The present invention relates to a rubber composition for tire tread and a pneumatic tire using the same. More specifically, the present invention relates to a rubber composition for tire tread having remarkably improved grip performance on wet road while maintaining processability and fuel efficiency, and a pneumatic tire using the rubber composition for the tread rubber.
In recent years, a wide variety of properties such as steering stability, abrasion resistance, riding comfort characteristics as well as fuel efficiency have been required for automobile tires and various ideas are suggested in order to improve these performances.
For example, it is tried to improve grip performance on road for the purpose of improving controllability and steering stability on wet road at high-speed running, or to improve cornering properties by increasing block stiffness of tire tread pattern and then preventing block deformation in case of cornering. It is also devised to inhibit deformation of groove so that excellent drainage is achieved to prevent hydroplaining.
Recently, grip performance on wet road is increased by compounding silica to a high styrene-content styrene-butadiene rubber (SBR) in order to meet the demand for such properties.
However, it is said that the above rubber composition for tire tread cannot exhibit sufficient grip performance on wet road or semi-wet road in high temperature range of over 15° C., though the grip performance can be increased in low temperature range of at most 15° C. Furthermore, it has been found that when running is carried on, the rubber composition containing silica shows decrease of rubber rigidity, resulting in remarkable decrease of grip performance. In addition, when dispersion of silica particles into rubber is insufficient, Mooney viscosity of the rubber composition is increased, causing problems that processability such as extrusion is poor.
In order to solve these problems, various suggestions have been made so far. For example, there are a rubber composition obtained by compounding sintered clay to a diene rubber; a rubber composition obtained by compounding vulcanized rubber powder comprising a diene rubber and kaolinite to a particular diene rubber; a rubber composition obtained by compounding an inorganic filler of a particular composition and carbon black to SBR containing a particular amount of styrene; and a rubber composition obtained by compounding clay comprising kaolinite as a main component to a diene rubber whose 1,2-bond content in butadiene part is in a particular range. It is now known that these rubber compositions have an effect on improvement of grip performance.
However, at present, no rubber composition has yet been produced, which has excellent wet grip performance while maintaining low heat build-up characteristics without lowering of processability and abrasion resistance.
In addition, both grip performance and rolling resistance (fuel efficiency) are properties related to hysteresis loss of a rubber. Generally, the larger the hysteresis loss is, the higher the grip performance is and the more improved the controllability is, but rolling resistance is increased at the same time, resulting in increase of fuel consumption.
In this way, grip performance and rolling resistance are incompatible, and therefore various rubber compositions for tire are suggested in order to achieve both properties simultaneously. For example, since polymers and carbon black have particular influence on both properties in the rubber composition for tire, it is tried to improve both the rolling resistance and the grip performance by suitably selecting the rate of combined styrene and the 1,2-bond content of butadiene part when a styrene-butadiene copolymer is used as a polymer. And in case of carbon black, the amount of carbon black is reduced or the particle diameter of carbon black is enlarged. In these methods, however, it is difficult to achieve compatibility between low heat build-up characteristics and reinforcing property, and abrasion resistance. Accordingly, carbon black whose activity degree of particle surface is optimized is used at present.
On the other hand, there are many reports on the method of using silica and a silane coupling agent in order to achieve low heat build-up characteristics. However, it has been found that, when running is carried on, the rubber composition containing silica shows decrease of rubber rigidity, resulting in remarkable decrease of grip performance. In addition, when dispersion of silica particles into rubber is insufficient, the rubber composition containing silica has increased Mooney viscosity, causing a problem that processability such as extrusion is poor.
For solving these problems, various suggestions have been made so far. For example, Japanese Unexamined Patent Publication Nos. 133375/1995 and 311245/1996 disclose a rubber composition obtained by compounding sintered clay to a diene rubber, Japanese Unexamined Patent Publication No. 3373/1996 discloses a rubber composition obtained by compounding a vulcanized rubber powder comprising a diene rubber and kaolinite to a particular diene rubber, describing effect on improvement of grip performance and the like. Moreover, Japanese Unexamined Patent Publication Nos. 59893/1996 and 59894/1996 disclose a rubber composition obtained by compounding an inorganic filler of a particular composition and carbon black to SBR containing a particular amount of styrene, while Japanese Unexamined Patent Publication Nos. 149954/1995 and 31250/1997 disclose a rubber composition obtained by compounding clay comprising kaolinite as a main component to a diene rubber whose 1,2-bond content in butadiene part is in a particular range, and similar effect is described.
In spite of these efforts, surface properties of inorganic fillers such as silica have a significant influence on rubber composition and often prevent the intended performance from being achieved to a remarkable degree. For example, since a silanol group, i.e., the surface functional group of silica forms a hydrogen bond, silica particles tend to coagulate with each other, causing problems such as lowering of mechanical strength and weakening of materials as well as remarkable decrease of workability.
In order to solve these problems, it has been tried to use various coupling agents, dispersion agents or surface modifiers. For example, it is considered that a sliane coupling agent combines to silanol group on the silica surface to prevent silica particles from coagulating with each other, resulting in improvement of processability. However, silane coupling agents are expensive, and can combine to some limited kinds of inorganic compounds such as silica, glass fiber and alumina owing to the characteristics of its functional group, and there has been a problem that the silane coupling agent is not effective for poorly reactive compounds such as titanium oxide, calcium carbonate, carbon black and graphite.
Other inexpensive dispersion agents or surfactants for inorganic compounds include an anionic, cationic or nonionic low molecular weight surfactant and fatty acid, but they have a problem that covering ability for inorganic compound is poor.
As mentioned above, the fact is that there is no rubber composition at present, which has excellent wet grip performance and improved dispersability of inorganic fillers while maintaining abrasion resistance and low heat build-up characteristics without lowering of workability and processability.